This invention relates to medical diagnostic ultrasound imaging, and more particularly to a method and apparatus for compounding ultrasound fields of view to acquire large ultrasound images.
Medical diagnostic ultrasound systems are commonly used to generate two-dimensional diagnostic images of internal features within a patient's body. To do so, a sonographer positions an ultrasound transducer probe adjacent to a patient's target area. The probe is a non-intrusive device including an array of acoustic transducer elements. The transducer elements emit ultrasonic energy at a frequency on the order of 2.0 MHz to 10 MHz. The transmitted ultrasound energy propagates into the patient whom it is in part absorbed, dispersed, refracted, and reflected by internal structures. Reflected ultrasound energy is received back at the transducer probe where it is converted back into electronic signals. Body tissues, for example, appear as discontinuities or impedance changes in the converted electronic signals.
Converted electronic signal samples undergo beamforming to correlate the samples in time and space to a patient's target area. Exemplary beamforming parameters for controlling the imaging process include focus, steering, apodization and aperture. Focus is a time delay profile of active transducer elements. Steering is the control of focus depth points along azimuth and elevation axes of a transducer probe scan. Apodization is a voltage weighting profile of active transducer elements. Aperture is the control of the number of transducer elements which am active along azimuth or elevation axes of the transducer probe for a given scan. The beamformed data are processed to analyze echo, doppler, and flow information and obtain a cross-sectional image of the patient's targeted anatomy (e.g., tissue, flow, doppler).
A conventional image is a brightness image (i.e., referred to as a `B-mode image`) in which component pixels are brightened in proportion to a corresponding echo sample strength. The B-mode image represents a cross section of the patient target area through a transducer's scanning plane. Typically the B-mode image is a gray scale image in which the range of lighter to darker gray-scale shades correspond to increasing brightness or echo strength. The typical ultrasound B-mode image is formed by a linear scan or sector scan of the patient's target area by the transducer probe. The individual images produced by ultrasound imaging systems include discrete frames. For a given scanning frame in which first active transducer elements transmit an ultrasound signal and second active transducer elements receive an ultrasound echo, the transducer probe defines a given field of view. Such field of view depends on the number of active transducer elements, the relative spacing of the elements and the steering and focus of each element. Each frame has a limited field of view due to a relatively narrow region traversed by the transmitted ultrasound energy. As the transducer probe is manipulated along the patient's body surface, each previous image is replaced off the viewing display by a new image defined by the current position, and thus field of view, of the transducer probe.
Given the generally narrow field of view of conventional ultrasound systems, it is desirable to extend the field of view to acquire images over large portions of the patient anatomy. Increasing the number of transducers is one approach. However, such approach adds significant hardware expense and processing overhead cost. Another approach is to compound images from the scanning process into a larger image. Previously, it has been demonstrated that a real-time compound ultrasound two-dimensional image could be generated by using so-called compound B-scanners. These B-scanners use a transducer mounted on an arm assembly that constrains the transducer to move along a single plane or axis. Either the arm assembly or the transducer element itself is provided with sensing devices for tracking the precise position of the transducer. This positional information then is used to register each one of discrete image frames into a composite image. An example of an arm assembly is disclosed in U.S. Pat. No. 4,431,007 to Amazeen et al. for Referenced Real-Time Ultrasound Image Display.
Systems and methods relying on hardware position sensors have several shortcomings for expanding the field of view. First, popular position sensors are based on electromagnetic energy emissions. An electromagnetic sensor, however, is to be avoided in an ultrasound system because the electromagnetic energy interferes with the transmitted and received ultrasound energy. Other hardware position sensors tend to be less accurate requiring longer and more frequent calibration processes. Also, it is a challenge to integrate the sensor's detection scheme into the ultrasound image capturing process. The position sensor captures data samples. Such samples need to be synchronized to the ultrasound sampling process and the ultrasound data processing data.
The hardware position sensor typically is located on the transducer probe. Thus the design of the probe (e.g., size and shape) is affected. This poses an ergonomic challenge in appeasing the sonographer. Sonographers tend to prefer limiting the gadgetry included on the probe. Accordingly, there is a need for expanding the ultrasound probe field of view without the use of a hardware position sensor. Specifically, for any given transducer array size, it is desirable to implement a method for expanding or compounding the field of view. Such method should be compatible with modern hand-held ultrasound transducers without encumbering the hand-held transducers with position sensing devices that increase the cost, weight and complexity of such imaging systems.